Hydraulic circuit for plural cylinders



l 1969 c. H. T. WOODWARD 3,464,317

HYDRAULIC CIRCUIT FOR PLURAL CYLINDERS 2 Sheets-Sheet 1 Filed July 28, 1967 INVENTOR. CYRIL WOODWARD Fay/A l 2" AGENT p 2, 1969 c. H. T. WOODWARD 3,464,317

HYDRAULIC CIRCUIT FOR PLURAL CYLINDERS 2 Sheets-Sheet 2 Filed July 28, 1967 ONT KKK

IN VENTOR. CYRIL HIWOODWARD PEZM AGENT US. Cl. 91-186 1 Claim ABSTRACT OF THE DISCLOSURE A hydraulic circuit for plural working hydraulic cylinders or motors which are operatively connected to actuate an apparatus or mechanism. This actuation may be by arms, roller chain orcable to an arm or sprocket on a shaft. The cylinders are arranged so that their dead ends are flow-connected to each other by a hydraulic bypass line and this line is filled from a feed line with high pressurized fluid so that the dead ends of the cylinders and connecting line is filled with pressurized fluid. A check valve in the feed line prevents a reverse flow from the bypass line to the supply of the pressurized fluid.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to hydraulic circuits or hydraulic feed lines as particularly found in the general class designated Motors, Expansible Chamber Type and particularly to the subclass designated plural working members, also to the subclass for double acting motors. Also pertinent is the general class of Expansible Chamber Devices and the subclass designated as plural unitarily mounted cylinders.

Description of the prior art The use of a pair of cylinders to operatively rotate a shaft is, of course, well known in the art and in particular is found in many patented hydraulic systems. A system for vehicular steering, for example, is shown in US. Patent 2,557,936 to Brown of June 26, 1951 while a fluid feed line system is shown in Parret, US. Patent 3,318,199 of May 9, 1967. These are exemplary of systems in which two cylinders are used to provide an actuation of the work movement. In these systems each cylinder has its working end flow-connected to a valve so that a high pressure supply is fed to one cylinder the supply of fluid is decreased in the working end of the other cylinder. With the piston rods of the cylinders operatively connected together by a chain, cable or other device they, the pistons, operate in response to the supply of fluid fed to and from each cylinder.

Prior to this invention the dead or unused ends of these cylinders were usually vented to atmosphere or were connected to a return line for discharge of the bypass fluid to the tank. The action of the conventionally connected cylinders to actuate the apparatus such as the rotation of the shaft does not include a means for stopping and locking the cylinders at a determined position.

\As for example, when a shaft is rotating or turning apparatus carrying heavy loads, particularly those that are unbalanced, the dead ends of the actuating cylinders may allow the cylinders to creep or bounce. This ocurs when the actuating flow to and from the cylinders is stopped by the controbvalves. In the present invention the con- United States Patent necting of the dead ends of the cylinders by a fluid bypass and supplying these ends and the bypass with fluid at high pressure insures that there is no fluid loss by the piston during its operation, and that the pistons stop immediately the controlling flow to and from the cylinders is shut off at the control valve or valves.

SUMMARY OF THE INVENTION The hydraulic circuit of this invention provides a means for controlling plural independent cylinders so as to work in complete unison with each other. These cylinders are preferably arranged in a substantially parallel alignment and have the ends of each piston rod connected to an arm, a roller chain, a cable or the like. The arm may be connected to a shaft and, when a chain, the chain is adapted to engage a mounted sprocket and the like so as to rotate a shaft upon which this sprocket is mounted. As the shaft often carries and rotates ofl? center and unbalanced heavy loads, it is desirable to have this shaft precisely controlled insofar as its rotation is concerned. To achieve this control the dead ends of the cylinder are connected by a bypass line to each other and to the high pressure source of fluid. In the supply line to the bypass line connecting the dead end of these cylinders there is provided a check valve to prevent a reverse flow from the bypass line to and through the supply line. The operating side of each of the cylinders is connected to a valve or valves so that as one or more cylinders are fed a determined or selected amount of high pressure fluid one or more other cylinders are opened to a discharge of their fluid. The power transmittting means such as the arm or chain tends to cause the cylinders to move in unison with each other. In addition, the opposite sides of the cylinders (defined as dead ends) are connected by the bypass line to each other and with the bypass line supplied with fluid at a positive high pressure. As the one or more cylinders are moved, a corresponding motion is induced in the other cylinders. When the valves controlling flow to an from the plural cylinders is stopped, the motion of the plural cylinders is stopped. The high pressure connection of the dead ends of the cylinders insures that no further travel of the pistons in the cylinders occurs.

In the usual manner of hooking up two cylinders to rotate a shaft a selected amount, it is customary to leave the cylinders with the dead ends connected only to a sump or drain so that as fluid is fed into the active side of a cylinder the dead end of this cylinder provides no restraint and may permit a small amount of travel or creep which is usually highly undesirable. In the hydraulic circuit of this invention there is provided a positive and complete union of the operation of one or more cylinders to one or more other cylinders so that the power transmitting means including the tautness of the chain controlled by the cylinders and the positive rotation of the shaft is precisely controlled in response to a valve or valves controlling the actuating fluid flow to the cylinders.

In certain of the applications of this invention it is desirable to have the effective force on the power transmitting lmeans be the force applied to the full piston face rather than the force usually available by connecting the rod-end side of the cylinders to the valve or valves. In this invention the full cylinder side is connected to the fluid flow control valve and the rod-end cylinder side is bypass connected to other rod-end sides of other cylinders. By further connecting this rod-end bypass to the high pressure fluid supply, it is insured that the dead end or rod-end side of the cylinders are filled with high pressure fluid. The fluid line from the high pressure fluid supply is further provided with a check valve to halt and insure that there is no reverse flow of fluid from the bypass to the high pressure fluid supply.

When the cylinders are activated the force applied to the full cylinder side of a cylinder is also the force developed on the opposite or rod-end side of this cylinder and transmitted by the bypass line to the rod-end side of another cylinder. This pressure in the bypass by virtue of the area differential between rod-end and full piston sides produces a pressure far in excess of the high pressure on the full piston side and also in the pressure line to the bypass. This excess pressure causes the check valve to close while this excess high pressure is provided on the rod-end piston side of the other cylinder. In effect, this causes the cylinders to exert a thrust equal to the piston area times the pressure although the thrust force is on the rod-end side of the cylinder.

The advantage provided by this circuit arrangement is tremendously important in nearly any hydraulic circuit with plural cylinders. The pressure from the pump need be calculated and provided based on the area of the full piston side. For example, if one thousand inch pounds of force is required, a full piston area of two inches requires a pressure of five hundred pounds per square inch. No matter what is the rod-end area of the piston, the pressure developed from the piston end is the amount of total pressure required. This permits a lower pressure to be supplied or used than if the rod-end of the piston is fed and controlled from the high pressure pump or line supply.

It is an object of this invention to provide a hydraulic circuit in which a plural arrangement of cylinders have their dead ends connected together by a fluid bypass and with this bypass fed a supply of pressurized fluid. After the bypass is full a check valve prevents the return of the fluid to the supply during the actuation of the pistons in the cylinders.

It is an object of this invention to provide a hydraulic circuit in which a plural arrangement of cylinders usually of like capacity and stroke are arranged so that from the rod-ends of the cylinders a power transmitting force such as an arm, a roller chain or the like is transferred to an arm, a sprocket or the like mounted upon a shaft having a load to be rotated. When connected to a sprocket this roller chain is maintained in a taut and controlled condition as the sprocket is rotated to rotate the shaft. A fluid bypass from the dead end or non-operable end of the first cylinder is connected to the second cylinder and a high pressure line feeding from the hydraulic supply or pump is connected to and positively maintains this line in a high pressure fluid full condition. This condition insures there is no leak of fluid past a piston or its rings and the like and as the one or more cylinders are moved by an inflow of pressurized fluid, a positive and like movement is provided in those other cylinders connected by the bypass so that as the pistons are caused to be moved, the shaft is positively rotated in response to the actuation and with the complete unison action of the cylinders resulting in a positive control of the rotation of the shaft.

It is a further object of this invention to provide a hydraulic system in which like ends of at least two cylinders are connected to each other by a fluid bypass, said bypass being provided with a high pressure fluid system to maintain the volume and pressure on the bypass connected ends of the cylinder.

It is a still further object of this invention to provide a hydraulic system in which the rod-end portions of a pair of cylinders are connected to each other by a fluid bypass and in which this bypass is provided with a fluid supply at a high pressure and a check valve is provided to insure that fluid in the bypass is not returned to the fluid supply.

It is a still further object of this invention to provide a hydraulic circuit or system in which the dead ends of two or more cylinder-s are connected by a fluid bypass and with the fluid bypass provided with a high pressure fluid supply, said supply having a check valve adapted to insure that fluid in the 'bypass is not returned to the fluid supply. These cylinders are arranged so that the fluid bypass connects ends of like or differing sweep volumes of a first group of one or more of the cylinders is moved by a hydraulic flow the other cylinders are positively moved an amount equal to the swept volume moved in the bypass by the movement of the first group of cylinders.

INTENT OF THE DISCLOSURE Although the following disclosure offered for public dissemination is detailed to insure adequacy and aid in an understanding of the invention, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how it may later be disguised by variations in form or additions of further improvements. The claims at the end hereof are intended as the chief aid toward this purpose, as it is these claims that meet the requirement of pointing out the improvement-s, combinations and methods in which the inventive concepts are found.

There has been chosen five specific embodiments of a hydraulic circuit for the complete unison action of plural hydraulic cylinders, these embodiments are chosen for the purposes of illustration and description of the novel hydraulic circuit of this invention and are shown in the accompanying drawings forming a part of the specification wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a somewhat schematic view of a hydraulic system of this invention in which the piston ends of a pair of cylinders are connected to a valve or valves for the input and release of hydraulic fluid to these piston ends, and the rod-ends of the cylinders are connected to each other by means of a fluid bypass line and this fluid bypass line is fed by a high pressure fluid supply and through a check valve with the lines connected so as to provide a positive and complete fluid connection of the rod-ends of the cylinders;

FIG. 2 represents a somewhat schematic view of a hydraulic system similar to FIG. 1, but in this arrangement the piston ends of the cylinders are fluid-connected by a bypass line to each other and this bypass line is fed by a high pressure line and through a check valve disposed so that the fluid in the bypass line is prevented from returning to the source and so that the piston ends of the cylinders are maintained in communication with a fluid supply and in which the rod-ends of the cylinders are connected to valves of the hydraulic system and through these valves pressurized fluid is applied to one piston and diminished on the other piston to cause the pistons to move in positive synchronism with each other; FIG. 3 represents a hydraulic circuit with three cylinders having their rod-end portions connected by a bypass line to each other and with this bypass line fed by a high pressure line and through a check valve disposed so that the fluid in the bypass line is prevented from returning to the source, two of the cylinders are arranged to act in tandem to perform an actuation and the other cylinder is arranged to actuate a rack and pinion mechanism;

FIG. 4 represents a hydraulic circuit as in FIG. 2 but in which the fluid bypass extends from a rod-end of one' cylinder to a piston end of another cylinder;

FIG. 5 represents a hydraulic circuit similario the circuit of FIG. 4 but with the cylinders at right angles to each other and performing separate actuations, and

FIG. 6 represents a hydraulic circuit for two cylinders arranged similar to the two cylinders of FIGS. 1 and 2 but of conventional prior art in which the rod-ends of the cylinders are each connected to a valve and the piston ends of the cylinder are connected to a sump or low pressure return line.

BRIEF DESCRIPTION OF THE PREFERRED- EMBODIMENTS Referring now to the drawings in which like numbers refer to like members throughout and in particular to FIG. 1, it is to be noted that in the diagrammatic showing of the hydraulic circuit a sprocket carried by a shaft 12 is rotated either clockwise or counterclockwise by means of a parallel arrangement of cylinders 14 and 16. These cylinders have their piston rods 18 and 20 provided with rod-ends 22 and 24 which are attached to and retain the ends of a roller chain 26. To the piston ends of cylinders 14 and 16 there is provided fluid feed lines 30 and 32 which are connected to a valve or valves not shown. These valves control the flow of pressurized fluid to and from the piston sides of the cylinders 14 and 16 so that as fluid is fed to one cylinder, it is released from the other cylinder.

The rod-end portions of cylinders 14 and 16 are provided with a bypass line fluid connection 34, which line is supplied by means of a line 36 with high pressure fluid from a pump or supply not shown. This high pressure fluid is often the high pressure supply used to operate the cylinders and in this line 36 there is a check valve 38 arranged to permit the flow of fluid to and through the line 34 so as to provide and maintain a fluid connection and pressure on the rod-ends of cylinders 14 and 16. This fluid supply insures that this closed circuit portion has a complete supply of fluid at a determined pressure which may be the pressure of the operating system.

OPERATION OF THE HYDRAULIC CIRCUIT IN FIG. 1

With the rod-ends of cylinder 14 and 16 connected to each other by means of fluid line 34 and supplied and filled with pressurized hydraulic fluid through line 36, it is obvious of course, that drive chain 26 is maintained in a taut condition by means of this pressurized fluid. It is also to be noted that with this closed hydraulic line filled with fluid and maintined under pressure, that as one piston is moved, the other piston is moved a like amount in a positive response. With both dead ends of the cylinders connected to each other with a complete fluid supply the check valve 38 insures that motion of one piston causes a like motion to the other piston.

The supply lines 30 and 32 are connected to a control valve or valves so that as line 30 is fed a quantity of pressurized fluid the line 32 is opened to a sump or the fluid reservoir and is under a diminished pressure so that the piston in cylinder 14 moves upwardly while the piston in cylinder 16 moves downwardly. The roller chain 26 maintained in a taut condition, of course, insures that the movement of the sprocket is precise and when the valves to feed line 30 and to feed line 32 are closed, the pressure and fluid on both sides of the pistons in cylinders 14 and 16 is fixed and with the pistons immobilized the rotation of sprocket 10' is stopped.

In like manner when the rotation of the sprocket is to be in a clockwise direction, the line 30 is opened to the sump or fluid resorvoir and line 32 is opened to the high pressure fluid supply so that the piston in cylinder 16 is moved upwardly and a like amount of movement of the piston in cylinder 14 is downwardly. The degree of rotation of the sprocket is precisely controlled by the movement of each of the pistons.

In the counterclockwise operation of the pistons in cylinders 14 and 16, it is noted that with line 30 opened to a supply of high pressure fluid, as the piston begins to move the fluid in the bypass 34 causes pressure to be applied to the rod-end of cylinder 16. The piston in cylinder 16 cannot move until line 32 is opened to discharge, whereupon the pressure on the rod-end of the piston in cylinder causes the chain 26 to move to rotate the sprocket 10 counterclockwise. The pressure times the area of the piston in cylinder 14 is the effective thrust on the rod-end of piston 16.

DESCRIPTION OF THE EMBODIMENT OF FIG. 2

In the hydraulic circuit of FIG. 2, the connections to the cylinders are opposite to the connections of the cyl inders of FIG. 1. The sprocket 10 carried by shaft 12 is engaged and driven by roller chain 26. This sprocket is rotated by the movement of the pistons in cylinders 50 and 52. The rods 54 and 56 have rod-ends 58 and 60 connected to the chain 26. The piston ends of cylinders 50 and 52 are fluid-connected to each other by means of a fluid bypass line 62 supplied with high pressure fluid through a line 64 and with the fluid line 64 having a check valve '66. The opposite or rod-end portions of cylinders 50 and 52 are operatively connected to the fluid feed lines 68 and and by means of a valve or valves not shown, the fluid flow to and from the lines 68 and 70 controls the rotation of the sprocket 10 and shaft 12.

OPERATION OF THE EMBODIMENT OF FIG. 2

With the piston ends of cylinders 50 and 52 being connected to each other by bypass 62 supplied by high pressure fluid through line 64 and check valve 66 the operation of the cylinders requires that the fluid lines 68 and 70 be connected to a valve or valves for a presurized fluid supply and return. This fluid supply may have the same high pressure as the pressure supplying the line 64. In order to move the sprocket 10 counterclockwise the line 70 is opened to the high pressure fluid and the line 68 is opened to the fluid return so that the fluid on the rodend side of the cylinder 50 is returned to the supply tank and the high pressure entering the rod-end of cylinder '52 through line 70 causes the movement downwardly of the piston in cylinder 52.

In like manner when the rotation of the sprocket 10 is to be clockwise, the line 70 is opened to the supply tank or low pressure line and the line 68 is opened to the high pressure supply so that fluid is fed into the rod-end of cylinder 50. The high pressure fluid initially in the piston end of this cylinder passes through the bypass line 62 and into the piston end of cylinder 52 to insure that the piston in this cylinder moves in direct relation to the movement of the piston in cylinder 50. When the valves controlling the flow to the lines 68 and 70 are shut, the pistons in cylinders 50 and 52 are immobilized with the fluid on both sides of the piston in a fixed condition. As the fluid on both sides of each of the pistons is under pressure and fluid flow is stopped, the movement of the pistons is stopped. In this manner the rotation of the shaft 12 is precisely controlled and stopped by the opening and closing of the valves.

With the fluid in the bypass 62 and the cylinders being under a determined pressure the opening of valves to provide a flow of pressurized fluid to line 70 requires line 68 to be opened to a return line or sump or no movement will occur. In like manner, the flow of pressurized fluid through line 68 requires line 70 to be opened to a return or no piston movement can be made. As the piston area on the rod-ends of cylinders 50 and 52 is less in area than the piston area on the other side of the piston, the bypass line may be kept at a pressure just below a pressure as computed by the formula Pressure (Piston side) Pressure (rod-end) Xeflective area (rod end) Area (Piston side) The pressure can be adjusted by conventional pressure reducing means so that with this determined pressure the check valve sure to close when the movement of the pis tons is made.

7 DESCRIPTION OF EMBODIMENT OF FIG. 3

The hydraulic circuit of FIG. 3 includes three cylinders 75, 76 and 77 connected at their rod-ends by a fluid bypass 78 and 79. This bypass is supplied with pressurized fluid through line 80 and reverse flow is prevented by check valve 82. Pressurized flow to the cylinders and from the cylinders to a drain is through feed lines 84, 85 and 86. The rods of cylinders 75 and 76 are joined by a bar 88 actuating link arm 89 and lever 90. The piston of cylinder 77 drives a rack 92 which actuates pinions 93 and 94 resulting in the movement of rack 95.

OPERATION OF EMBODIMENT OF FIG. 3

As cylinders 75 and 76 are contemplated to be moved in unison, the feeding of fluid through leads 84 and 85 and the opening of line 86 to the return will result in the piston of cylinder 77 moving downward at about twice the upward rate of cylinders 75 and 76. Conversely, the feeding of fluid to cylinder 77 through line 86 and the opening of lines 84 and 85 to the return will result in the piston of cylinder 77 moving about twice the speed and distance of the pistons of cylinders 75 and 76. The pressure in bypass lines 78 and 79 will be higher than the fluid pressure in feed lines 84, 8 and 86.

DESCRIPTION OF THE EMBODIMENT OF FIG. 4

The hydraulic circuit of FIG. 4 shows an arrangement in which a pair of cylinders 100 and 101 is mounted so that a connected fluid bypass line 102 extends from the rod-end of cylinder 100 to the piston end of cylinder 101. This line 102 is fed pressurized fluid through line 104 and check valve 105. Fluid is fed to and from the piston end of cylinder 100 by means of line 106. The fluid to and from the rod-end of cylinder 101 is through line 107. Cylinder 100 moves link 109 While cylinder 101 moves link 110 and arm 111.

OPERATION OF THE EMBODIMENT OF FIG. 4

The feeding of fluid to line 106 advances the piston in cylinder 100 and with line 107 open to the drain the piston in cylinder 101 is moved and at a lesser speed and for a lesser distance, that is assuming the pistons are the same size. With cylinder 101 fed fluid through line 107 and with line 106 open to the drain, as the piston in cylinder 101 moves upwardly the piston in cylinder 100 moves at a greater speed and distance. Links 109 and 110 are merely representative of actuations which may be performed by the cylinders.

DESCRIPTION OF EMBODIMENT OF FIG. 5

The hydraulic circuit of FIG. 5 is the same as that of FIG. 4 but it is to be noted that cylinders 120 and 122 are shown at right angles to each other to illustrate that the arrangement of cylinders need not be at any particular relation to each other and their actuations may perform operations on the same or differing mechanisms. Although bypass 124 is shown as connecting the rod-end of cylinder 120 to the piston end of cylinder 122 the reverse could as well be made or, if desired, like ends could also be connected as in FIGS. 1 and 2.

OPERATION OF THE EMBODIMENT OF FIG. 5

The operation of FIG. 5 is the same as that of FIG. 4 except that it is to be noted that the connection of the bypass regulates the speed of operation and the effective work capacities that are possible. The providing of a closed bypass circuit for the positive movement of the cylinders and the effective pressure ratios are matters of selection. In like manner the size of cylinders is governed by the use and the intended fluid pressure. The size and disposition of the cylinders are merely a matter of selection.

DESCRIPTION OF THE PRIOR ART (FIG. 6)

Referring finally to FIG. 6, there is shown a conventional or prior art hydraulic circuit for a pair of parallel cylinders arranged as in FIGS. 1 and 2. As shown, the rod-ends of the cylinders are actuated by a regulated fluid supply. Sprocket 10 carried by shaft 12 and rotated by chain 26 is actuated by means of cylinders and 132 having rods 134 and 135. The rods 134 and 135 carry ends 137 and 138 attached to the chain 26. Fluid feed lines 141 and 142 and drain lines 144 and 145 are connected to the cylinders 130 and 132.

OPERATION OF THE CONVENTIONAL PRIOR ART (FIG. 6)

It is to be noted, of course, that the actuation of the cylinders 130 and 132 is through lines 141 and 144 which are connected to the valves (not shown) controlling the feeding of high pressure fluid to and from the rod-ends portion of the cylinders. The other sides of these cylinders are connected by lines 142 and 145 to the return tank. This operation, of course, is conventional. However, it is to be noted that with the lines 141 and 144- closed, a small amount of fluid may leak past the pistons. This leak or a delay in closing the valves may result-in a differential in pressure on the rod-end portions of the cylinders. Any differential provides a small amount of resilience or a small amount of bounce or slippage to occur within the pistons. This results in a creep motion which is highly unsatisfactory particularly if the shaft 12 carries a load which is unbalanced. As any fluid bypass is drained from the cylinders 130 and 132 through lines 142 and 145 this fluid is not replenished until the control valves are opened. Such a system or hydraulic circuit lacks the positiveness or unison action which is provided in the hydraulic circuits of FIGS. 1 through 5 above-described.

It is of note that a condition which may and does occur in the circuit of FIG. 6 exists when the fluid pressure to the system is provided by a pump or the like which pressurizes the fluid only at the time of use. With no pump pressure or the pressure shut 01f, the inadvertent opening or leaking of the valve or valves controlling lines 141 or 144 allows an unbalanced load to move one piston without a like movement in the other piston. This allows a slack to develop in the chain and the chain can slip on the sprocket and accidents can and have happened. The positive control of FIG. 1 prevents any slack to develop in the chain and with the fluid pressure in the bypass controlled by the check valve, any accidental opening of valves still does not permit the bypass to drain and the chain to slacken.

DEFINITIONS The term dead end as used in the above description refers to that portion of the cylinder which is not connected to a valve through which pressurized fluid is fed or the residual fluid in the cylinder is discharged. The terms rod-end and piston end are felt to be conventional and appropriate as applied; rod-end refers to that side of the cylinder containing the piston rod, while piston end" refers to that side of the cylinder having no rod.

Terms such as up, down, clockwise, counterclockwise and the like are applicable to the embodiments as shown and described in conjunction with the drawings. These terms are merely for the purpose of description and do not necessarily apply to the apparatus actuated by the hydraulic circuits and the manner in which they may be constructed or used.

What is claimed is:

1. A hydraulic circuit for use with a pair of cylinders, each having a rod end and a piston end, the rod ends connected to each other by a flexible drive member such as a roller chain, the cylinders arranged to transmit by said flexible member a power actuation to a sprocket carried on a shaft and the like, the circuit disposed to maintain the flexible member at a determined tension to prevent slack from developing in said flexible drive member, said circuit including: (a) a first fluid conductor connected to the piston end of the first cylinder; (b) a second fluid conductor connected to the piston end of the second cylinder; (c) a third fluid conductor connected to the rod end of the first cylinder; (d) a fourth fluid conductor connected to the rod end of the second cylinder and also to the third fluid conductor; (e) a supply fluid conductor connected to said third and fourth fluid conductors, an (f) a check valve in the supply fluid and prevent the return of any of the initial supply of pressurized fluid fed to the third and fourth fluid conductors and the rod ends of the pistons after said rod ends of the pistons and the third and fourth fluid conductors are filled with fluid at a determined pressure, the initial supply of pressurized fluid under pressure in said third and fourth fluid conductors being maintained by said check valve References Cited UNITED STATES PATENTS 3,255,587 6/1966 London 91-468 3,267,816 8/1966 Graham 91-186 3,267,817 8/1966 Adams 91--186 FOREIGN PATENTS 917,623 2/1963 Great Britain. 1,147,488 4/ 1963 Germany.

PAUL E. MASLOUSKY, Primary Examiner US. Cl. X.R.

absent a supply or maintenance of pressure in the supply 15 91-468; 92 68 131 fluid conductor prior to the check valve.

CERTIFICATE OF CORRECTION I Patent No. 3, l6 &,317 Dated September 2. 1969 Inventor (I) C .H.T WOODWARD It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 6, line 7, after "valve insert --is--;

Col. 9, line 7, an" should be --and-;

Col. 9, line 8, before "and" insert --conductor, said check valve disposed so as to maintain--.

SIGNED AN'U SEALED DEC 9 1959 Attest:

Edward M. FletcherJrwnmm E. sum JR. Atteating Officer Gomissioner of Patents 

